In motor vehicle engineering, “brake-by-wire” brake installations are being used ever more widely. Such brake installations comprise not only a master brake cylinder which is actuatable by the vehicle driver but also an electrically activatable pressure provision device, by means of which an actuation of the wheel brakes is performed in a “brake-by-wire” operating mode, in which the vehicle driver is decoupled from direct action on the wheel brakes.
Brake installations which are intended for use in vehicles which drive in automated fashion must, for a fault which occurs during automated driving (autopilot), provide an electrically controllable fall-back level in the brake installation, which ensures reliable braking of the vehicle until the human vehicle driver has taken over the task of driving again. In this regard, it is known from DE 10 2014 225 954 A1, incorporated herein by reference, to provide a brake installation having a primary brake system, which comprises a master brake cylinder actuatable by means of a brake pedal, a connectable and disconnectable simulation device for generating a brake pedal feel as required, a first electrically controllable pressure provision device for actuating the four wheel brakes, and an electrically controllable pressure modulation device for setting wheel-specific brake pressures for the four wheel brakes, and having a secondary brake system for the two front-wheel brakes, which secondary brake system is connected hydraulically in series between the primary brake system and the two front-wheel brakes. The secondary brake system comprises a second electrically controllable pressure provision device for actuating the front-wheel brakes and, for each front-wheel brake, at least one electrically actuatable isolating valve, which is arranged in the hydraulic connection between the first brake system and the corresponding front-wheel brake. In the event of a failure of the primary brake system, the secondary brake system constitutes the electrically activatable fall-back level for the braking of the vehicle during automated operation. Here, in DE 10 2014 225 954 A1, it is described that, in the normal operating mode of the brake installation, the four wheel brakes are supplied with brake pressure by means of the first pressure provision device of the primary brake system. In the event of a failure of the first pressure provision device or of the primary brake system, only the two front-wheel brakes are supplied with brake pressure by means of the second pressure provision device of the secondary brake system.
This constitutes the electrically controllable fall-back level for the braking of the vehicle during automated operation. DE 10 2014 225 954 A1 describes an operating concept for the brake installation in which either the primary brake system or the secondary brake system in each case exclusively (on its own) implements the braking demand of a virtual driver (autopilot). Here, “exclusively” means that, while one brake system performs the pressure provision, the other brake system is passive or “electrically deenergized”, that is to say neither the pressure provision device nor any of the valves of the other brake system is actuated. Such an operating concept however has numerous disadvantages. For example, a superposed brake actuation by the human driver during the provision of pressure by the secondary brake system, that is to say a takeover by the driver, cannot be performed in a manner controlled by the system. Also, during the provision of pressure by the secondary brake system only to two of the four wheel brakes, the possible braking action is limited to approximately 60%.